Two stage powder application method

ABSTRACT

A two stage powder coating method involves applying relatively large size powder particles to a surface to be coated during a first coating stage, followed by applying relatively small size powder particles to the large size particles during a second coating stage. The particles may be heated to cause melting after the first and/or the second stages, thereby to level the coating. This two stage powder application method assures a level, uniform and aesthetically pleasing finish for the applied powder coating, even for surfaces with hollow and/or recessed portions.

FIELD OF THE INVENTION

This invention relates to an improved method for powder coating.

BACKGROUND OF THE INVENTION

In many powder coating applications, the average particle diameterapplied to the surface to be coated varies depending on the thickness ofthe coating. For instance, if a relatively thick coating is desired,particles having a thick diameter are used. If a relatively thin coatingis desired, particles having a thin diameter are used.

However, with large sized particles, the applied coating or film doesnot always appear level and smooth, even if heating is used to coalescethe particles after contact on the surface. This poor leveling of largeparticles is particularly acute when the coating material is athermosetting polymer. In some cases, the coating looks uneven, like theexternal surface of a lemon, an effect sometimes referred to in theindustry as the "orange peel" phenomenon.

In contradistinction, leveling is more easily achieved with smallersized particles. However, per particle, the ratio of surface area toweight becomes high, and bulk density drops. When the particles areapplied by spraying, the effect of air resistance makes it difficult tomaintain a straight flight path for long distances. The ability tomaintain a straight flight path is referred to as linearity. For coatingobjects with hollow or recessed portions, penetration of the smallpowder particles to these surfaces is poor. This poor coveragenecessitates an increase in powder ejection air pressure. However, thisincreased ejection air pressure results in increased flow volume, sothat while this additional air pressure carries the particles farther ittends also to carry particles away from the surface when the transportair rebounds away from the surface.

Additionally, powder having a relatively small or fine particle size hasa high manufacturing cost. Thus, despite its well known coatingqualities, the industry has generally not used powders of relativelysmall size particles.

It is an objective of this invention to improve the leveling and theaesthetic appearance of a powder coating applied to a surface, withoutcompromising the delivery of the powder particles to the surface beingcoated or the adhesion of particles to the surface.

SUMMARY OF THE INVENTION

The present invention achieves the above-stated objectives by a two stepparticle application method which involves initially applying large sizeparticles to a coating surface, followed by applying small particles tothe large size particles. The average particle diameter for the powderapplied during the first stage (Stage A) is 15 to 120 microns, and theaverage particle diameter for the powder applied during the second stage(Stage B) is 0.1 to 15 microns.

Preferably during both stages, the particles are applied by spraying.The larger sized particles applied during the first stage may betemporarily heated to cause partial melting and increased adherence.Subsequently, when the second stage particles are applied, heat mayagain be used to melt the applied powder particles. Stages A and B maybe repeated as necessary, to achieve the desired coating thickness.

One advantage that results from this two stage powder application methodis increased adhesion efficiency. Also, by initially applying the largersized particles, the desired film thickness can be achieved, whilesubsequent application of the smaller sized particles improves overallleveling and surface appearance.

These and other features of the invention will be more readilyunderstood in view of the following detailed description and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view which schematically shows a powdercoating achieved with the two stage powder application method of thisinvention, wherein the coated surface is flat.

FIG. 2 is a cross-sectional view, similar to FIG. 1, which schematicallyshows a powder coating achieved with the two stage powder coating methodof this invention, wherein the coated surface has a recess.

FIG. 3 is a cross-sectional view, similar to FIGS. 1 and 2 wherein thepowder particles applied during the first stage are heated to achievemelting on the surface before the second stage of finer particles isapplied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts a surface 11 to be coated, and in this case the surface11 is flat. The two stage powder application method of this inventioninvolves initially applying to the surface 11 a powder 12 having arelatively large particle diameter. Preferably, the powder 12 has anaverage particle diameter in the range of about 15 to 120 microns.Subsequently, a powder 13 having a relatively small particle diameter isapplied. Preferably, the powder 13 has an average particle diameter inthe range of about 0.1 to 15 microns. Thus, the large particles 12 areapplied during a first stage, while the smaller particles 13 are appliedduring a second stage. Preferably, the powder applied during the firstand second stages are applied by spraying.

Because of the large particle diameter used in the first stage, thedesired film thickness can be achieved. Subsequent application of thesmall particles 13 improves overall leveling and surface finish of thecoating on the surface 11.

FIG. 2 illustrates another advantage of this two stage powderapplication method, an advantage which relates particularly to coatingobjects or articles which have an external surface with one or morehollow or recessed narrow portions, such as steel tools. Coating ofobjects such as this increases their resistance to rust. These objectsusually also have one or more external surface sites which are morenoticeable to consumers and thereby must have a favorable aestheticappearance. A favorable appearance of this type generally requirescoating with relatively small diameter particles. The two stage powderapplication method of this invention achieves both of these objectives,uniform coating of recessed narrow portions and an aestheticallypleasing coating on the external surface sites which are most noticeableto the consumer.

More particularly, FIG. 2 illustrates a portion of the surface 11 whichhas a recess 15 formed therein. Again, the coating on the surface 11comprises an initial application of relatively large particles 12followed by an application of small particles 13. The relatively largeparticles 12 used in the first stage of this method assure uniformcoating within the recess 15, since the larger particles can penetrateinto the recess to be deposited thereon, while the relatively smallparticles 13 level the overall coating of the entire surface 11 andfurther assure an aesthetically pleasing coating at the most noticeablesites.

One advantage of this method of powder coating is that in someapplications where the coated object is subsequently processed, as inthe bending of steel plates, cracks may occur in the coated film duringthis subsequent processing. However, with the two layers of coatingapplied according to this method, the bottom layer of large particles isrelatively thick and has good workability and adhesive properties, whilethe top layer of small particles is formed of powder made of materialswhich are tough and have desired properties, such as chemical and/orweather resistance.

Another advantage of this invention relates to achieving a matted tone,a suede tone or a hammer tone coating for an article. These are designproperties which have recently become popular in the market. By using,during the second stage, particles composed of urethane, polystyrene orglass beads which may be colored, it is easy to achieve a film with amatted or suede tone. Also, by using second stage particles which have aresin with poor compatibility with respect to the bottom layer, a hammertone film can be achieved.

In the application of large particle diameter powders, and particularlythose with a high molecular weight, it is difficult to achieve goodlevelling during heating and melting as noted above. The air present inthe interstices between particles impairs levelling, and if the coatedsurface is too rapidly melted, the air surrounding the particlesproduces air bubbles. To solve these problems, this two stage powderapplication method involves temporarily heating the large size particlesto melt them and subsequently applying the small size particles to coverthe voids or pinholes in areas where the coating is thin or packed byair bubbles. This makes it possible to obtain a high quality film withno pinholes or voids and to achieve favorable levelling. The appearanceof such a film is shown in FIG. 3, where reference numeral 16 designatesthe initially applied large particle diameter powder which is in amolten state due to temporary heating.

The invention contemplates the use of various types of powder. It ispreferable to use an organic powder during the first stage, due tobetter adhesive properties. However, a mixture of inorganic and organicpowders or a powder in which an organic powder is surrounded by organiccapsules may also be used. This invention results in a high performancepowder coating with relatively low costs, particularly when usingpowders such as fluorine and polyamide which are relatively expensive.Additionally, with a powder made of thermosetting material, due todifferences in the catalyst, the hardener or the functional group used,compatibility may be poor and/or a phenomenon referred to as "crawling"may occur. Therefore, it is preferable to use powders composed of thesame resin during Stage A and Stage B.

During powder coating, the phenomenon of levelling depends to a largeextent on the surface tension during the melting of the powdermaterials. It has been found that a surface tension of 40 dyne/cm ispreferred during melting of the powder. To maximize cost savings andproductivity, it is desirable to use relatively large amounts of thelarge particle diameter powder and to increase the amount used duringStage A of particle application, since this powder is relativelyinexpensive compared to the fine powder.

Also, by applying successive layers of powder having a large particlediameter and composed of a resin having a functional group and a smallparticle diameter powder composed of a hardener, the overall performanceof the applied film can be improved. For example, this can be achievedby repeated application in overlapping layers or dispersion applicationof large diameter particles comprising a mixture of polyester and anisocyanate functional group and small diameter particles in which blockisocyanate is encapsulated, followed by heating and melting or crosslinking.

For each of these variations of the invention, this two stage powderapplication method produces a film or coating with favorable levelling,excellent aesthetic appearance, good linearity of the powder duringapplication, good adhesion efficiency and high product quality.

While several preferred embodiments of the invention have been shown anddescribed, it is to be understood that variations in these modificationsmay be made without departing from the scope of the invention.Accordingly, applicant wishes only to be bound by the claims appendedhereto.

We claim:
 1. A method for powder coating a surface comprising the stepsof:a) applying to the surface, during a first coating stage, a firstlayer of powder particles having a first average diameter; and b)applying to the first layer of powder particles, in a second coatingstage subsequent to the first coating stage, a second layer of powderparticles having a second average diameter smaller than the firstaverage diameter so that at least some of the particles applied duringthe second coating stage fill spaces between particles applied duringthe first coating stage, thereby to form a coating on the surfacecomprising powder particles applied during the first and second stages.2. The method of claim 1 wherein the first average diameter is in therange of about 15 to 120 microns and the second average diameter is inthe range of about 0.1 and 15 microns.
 3. The method of claim 1 andfurther comprising the steps of:prior to the second coating stage,heating the powder particles applied during the first coating stage tocause melting thereof, and subsequent to the second coating stage,heating all of the powder particles to cause the particles appliedduring the first and second stages to melt and subsequently harden toform a coating.
 4. The method of claim 1 wherein the powder particlesused in the first and second coating stages, comprise the same resin asa main component.
 5. The method of claim 2 and further comprising thesteps of:repeating steps a) and b) to achieve a coating having a desiredthickness.
 6. The method of claim 1 wherein the powder particles areapplied in the first and second coating stages by spraying.